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On Oct. 27, 2010, a wintery blast buried Salt Lake City in 6 inches of snow, and dumped more than twice that in the Wasatch mountains. But that early-season storm could have packed an even bigger load had it not been for the mountainous topography northwest of the Great Salt Lake, says a new study that used computer simulations to illuminate northern Utah's "lake-effect" snowfall.

The Jarbidge-Caribou highlands of Nevada and Idaho actually deflect storm-bearing air masses from the Pacific Northwest, muting the lake effect, according to University of Utah atmospheric scientist Jim Steenburgh. But because of downwind ranges, moisture-laden weather still gets concentrated in the Salt Lake Valley and up Wasatch canyons where skiers reap the powdery bounty.

The Great Salt Lake has enhanced from three to 20 storms each winter since 1998, but the effect is spotty, felt in some places and not in others. It accounts for 5 to 8 percent of precipitation east and south of the lake from September to May.

Lake effect occurs when cold air travels over warmer bodies of water, sucking up moisture and depositing it as storms climb over higher terrain. The Great Lakes are responsible for powerful lake-effect snow storms in the Northeast, while the world's biggest effects are seen in Japan, whose mountains get pounded by weather crossing the Sea of Japan from the Asian mainland.

But the U.'s study demonstrates that the phenomenon results from a complex interplay of many factors  not just cold air passing over water.

"It tuns out you need something to get the precipitation organized," said Steenburgh, senior author on the study published Tuesday in the American Meteorological Society journal Monthly Weather Review. That something is usually mountains, which cover the Great Basin in abundance.

Steenburgh's co-author is doctoral candidate Trevor Alcott, who has since graduated and works for the National Weather Service (NWS) in Salt Lake City.

The NWS and the National Science Foundation funded the study.

In Utah's case, storms tracking from the Northwest get diverted up the Snake River plain before they can reach the Great Salt Lake. The air warms and dries as it goes over the Raft River Range and other mountains west of the lake, the study reported.

Research by Steenburgh and his associates is helping local forecasters improve their predictions concerning lake effect snow bands, according to Randy Graham, the NWS' science operations officer in Salt Lake City.

"It hard to predict where a band will form and how intense it might be. It's harder on the Great Salt Lake here than in the Great Lakes region. That's because of the topography. It results in more complex flows," Graham said. Steenburgh and Alcott "found that it can lead to some drying of low-level air masses. One challenge here is we don't have a lot of observational data from the Northwest, upwind of the lake."

Steenburgh, an avid skier who expounds on snow in his Wasatch Weather Weenies blog, agrees that predicting lake effect is tricky.

"The lake effect is a hard problem to solve. A small change in wind can play a big role in where the lake effect occurs," he said. Clouds bearing the lake's water move along narrow channels as they are shepherded south between the Oquirrh and Wasatch ranges that frame the Salt Lake Valley.

"It organizes into a single band about 10 to 15 miles wide. We have no way to forecast where that will happen until it happens," Steenburgh said. "We are getting better at predicting whether lake effect will occur, but how big it will be and where it will be is still a big challenge."

Without the influence of these downwind ranges, which act like a weather funnel, the October 2010 storm would have been a bust.